An optical disc apparatus has been developed to provide high density storage of information and high speed transfer rate of information. For instance, an optical disc apparatus to be adapted to a Hi-vision television system having an information amount which is five times greater than that of a conventional NTSC television system, and an external storage apparatus for computer peripheral having high speed and access properties of a magnetic disc used for a computer and a large capacity memory property of an optical disc have been intensively researched and developed. Disc apparatus for the next generation is expected for an optical disc apparatus in consideration of the necessity of high speed transfer rate and large capacity memory. Especially, it is required that the weight of an optical head itself is reduced to provide high speed access.
The basic structure of an optical head is mainly contructed in one-beam system or three-beam system. It is said that each of them is advantageous in one aspect, and disadvantageous in the aspect. However, the one-beam system is considered to be advantageous in that no dispersion light is observed in recording information, although it depends on a sensitivity of a recording medium. In a recording and reproducing type of an optical head, the maximum output power of a laser which is available at the present time is approximately 35 mW.
Conventional optical heads using one-beam system is described in the Japanese Patent Kokai Nos. 60-129943 and 62-88145.
In the conventional optical heads, a first conventional apparatus for detecting a servo signal of an optical head comprises first and second four-divided optical sensors for focus and tracking error detections, and a second conventional apparatus for detecting a servo signal of an optical head comprises a six-divided optical sensor of focus and tracking error detections. The detail of the first and second conventional apparatus will be explained later.
In the first and second conventional apparatus for detecting a servo signal of an optical head, however, there are disadvantages in that a focus error signal of high precision is difficult to be obtained, because a tracking error signal is leaked into the focus error signal, and stability and reliability of providing a six-divided optical sensor are low, because the divided photodetector regions are required to be adjusted by the order of .mu.m.